Powder metallurgy produced valve body and valve fitted with said valve body

ABSTRACT

The invention relates to a valve body produced by powder metallurgical methods that exhibits high thermal and wear resistance and has the following composition by weight: 
     0.5% to 2.0% C; 5.0% to 16% Mo; 0.2% to 1.0% P; 0.1% to 1.4% Mn; 0% to 5% Cr; 0% to 5% S; 0% to 7% W; 0% to 3% V, &lt;2% of other elements with the remainder being Fe. 
     It also relates to a valve fitted with such valve body.

The invention relates to a valve body produced by powder metallurgicalmethods that exhibits high thermal and wear resistance and to a valvefor combustion engines fitted with said valve body.

Inlet and outlet valves for combustion engines must satisfy highrequirements with respect to thermal endurance and wear resistance.Especially for the high-compression engines of multi-valve andelectronic control design it has been more and more difficult to findmaterials that on a long-term basis meet the needs linked with the hightemperatures arising at the outlet. Valves manufacture has become moreand more sophisticated and expensive and the material and machiningcosts involved in the process reflect this.

For the production of valve bodies or entire valves powder metallurgicalmethods have been proposed occasionally. Such powder-metallurgicalprocesses have been used in the manufacture of valve seat inserts buthave not yet gained widespread acceptance in the production of valvebodies or valves proper. Reasons for this were an insufficientdurability of the materials and their inadequate performance in terms ofthermal resistance.

To enhance the performance of conventionally manufactured valvesinductive hardening or armoring methods have been employed to processparticularly stressed areas—particularly for the valve seat. Thismeasure aims at keeping wear within acceptable limits with considerationto be given to the fact that even with this technology valvetemperatures of between 800° C. and 900° C. should not be exceeded.Nevertheless, it is increasingly difficult to meet this requirement withpresent-day engines.

Producing valves and valve bodies by conventional processes has becomeextremely complex, particularly with respect to seat armoring. Startingout with a rod segment the valve body is produced initially by heating,upsetting, calibrating and turning, to which then a rod segment isattached by friction welding. Additional working steps comprisestraightening, turning, grinding, and build-up welding, grinding andheat treatment to produce the finished valve with seat armoring.Especially, the seat armoring step involving build-up welding may leadto faults associated with unacceptably high reject rates.

Solutions aimed at providing a suitable seat armoring in the form ofpowder metallurgically produced armorings were sought but could notreach a mass production status. Applying the seat armoring did not bringabout a reduction of the failure rate. Such an armoring produced bypowder metallurgical methods rather turned out to be susceptible tocrack formation during subsequent processing steps.

With respect to the materials and additional processing steps requiredto produce armored or hardened seats it appears to be desirable to makevalve bodies of a homogeneous material in as few steps as possible withthe material having the necessary wear resistance, service life and heatdissipation characteristics and with the body being connected to a rodthus forming a valve.

Methods for the production of at least the wear lining of high-dutysintered parts in connection with the control of valves in an internalcombustion engine are known from DE 41 04 909 A1. The sintered partsthus produced by powder metallurgical steps feature a high chromium andcarbon content and are employed as cams for valve control purposes.Using such sintered parts for valve bodies has not been envisaged.

For the production of sophisticated shaped parts powder metallurgyfrequently offers advantages over conventional techniques in that thematerial characteristics can be optimized and the number of processingsteps reduced.

Therefore, it is the object of the invention to produce valve bodies forvalves by powder metallurgical methods from a material suitable for thepurpose, particularly with a view to reducing the manufacturingexpenditure. This method shall render a seat armoring dispensable andthe valve, respectively the valve body shall feature thermalconductivity properties sufficient for temperature control. The valvebody shall be connected with a conventionally manufactured valve rod bybutt-joining methods thus forming a functioning and durable valve.

This object is achieved by providing a powder metallurgicallymanufactured valve body having the following composition by weight:

0.5% to 2.0% C; 5.0% to 16% Mo; 0.2% to 1.0% P; 0.1% to 1.4% Mn; 0% to5% Cr; 0% to 5% S; 0% to 7% W; 0% to 3% V; <2% other elements, theremainder being Fe.

The invention also relates to valves manufactured with such valvebodies.

The metal powder used according to the invention is characterizedparticularly by a rather high content of carbon, molybdenum, andphosphorus.

The carbon and phosphorus constituents bring about the formation oftemperature resistant and wear reducing carbide and phosphide phasesthat lend to the material the service life span required. Chromium,vanadium and tungsten may be added to enrich or vary the propertyspectrum but, especially for the production of valves and valve parts,are not necessarily needed. An appreciable content of sulfur, especiallywhen present in the form of MoS₂, may serve as an internal lubricant butas a rule is not required for valves and components.

Valve bodies produced by powder metallurgical methods according to theinvention may be manufactured by conventional pressure sinteringprocesses. These include hot isostatic pressing although the process isnot necessarily required. Normally, a compaction of 7.5 g/cm³ issufficient even though a higher density, in particular appr. 7.7 g/cm³or higher, offers significant benefits. By increasing the density andthus causing the pore volume to be reduced the heat conductivitycharacteristics and temperature behavior are improved. Moreover, thevalve service life is improved.

The valve bodies according to the invention can be produced by therespective element powders. However, it will usually be expedient to usecompletely alloyed constituents for the manufacturing process, forexample, a completely alloyed steel component, a phosphorus-molybdenumsteel, possibly MoS₂, and, if necessary, graphite as additive, each inpowder form. An especially preferred embodiment is the use of metalpowders of irregular particle form produced by atomizing processes thatdue to their interlocking capability may lend a certain inherentcohesion to the pressed part produced from these substances. To improvethe workability, reduce wear in presses and enhance the cohesioncapability of the substances auxiliary agents may be used as additives,for example wax, up to an amount of 1% by weight based on the alloyingpowder.

Preferably, dendritic or spattering powders having a mean diameter ofless than 150 μm are put to use, preferably of a size of less than 50μm. Most expediently, carbon is to be admixed in the form of graphitehaving an average grain size of 10 μm or less unless contained in thecompletely alloyed powder in sufficient quantity. The PMoFe steel powderas can be used for the purpose outlined here has been described inWO-A-91/18123.

An especially preferred powder composition for the manufacture of valvebodies consists of 0.5 to 2.0% carbon, 5.0 to 14% molybdenum, 0.2 to1.0% phosphorus, 0.1 to 1.2% manganese, with a maximum of 0.50% ofchromium and maximum of 0.40% of sulfur. Other elements in this case arepresent in the amount of less than 2% with the remainder being iron. Thecomposition indicated reflects percentages by weight.

For the valve bodies according to the invention it is recommendable touse the liquid-phase sintering process. The finished valve body shouldhave a density of at least 7.7 g/cm³.

As compared with the conventional manufacturing process for completevalves described above the inventive powder metallurgically producedvalve bodies and the valves manufactured with such bodies offer asignificantly reduced number of processing steps. In the production of avalve according to the invention using a separately manufactured valvebody and a rod segment the following steps are involved:

At first pressing and tempering of the valve body followed by theprovision of the rod segment, joining together valve body and rodsegment, for example by friction welding, straightening, turning,grinding and heat treating the completed valve. As a result of thesignificantly decreased number of production steps the manufacturingaccuracy is improved and the failure probability brought down. Moreover,the reduced number of production steps makes it possible to react moreflexibly to changing system requirements.

The valves and valve bodies according to the invention feature high wearresistance even at elevated temperatures and high loads in the valvegear mechanism, particularly for outlet valves.

With respect to the valves proper the valve body consists of the abovementioned materials. The shaft is produced by conventional methods, i.e.without powder metallurgical processes, employing a conventionalmaterial. Valve body and valve rod are connected by means of abutt-joining method. For such a butt-joint connection a friction weldingprocess is preferred, however, other joining processes may be employedas well.

As far as the invention relates to valve bodies these offer benefitsover conventionally produced valve bodies in that they consist of ahomogeneous material and thus do not need to be modified locally to suitthe special configuration of a piston outlet in an internal combustionengine. Apart from advantages associated with the production processesthis reduces susceptibility to failures and defects in the product bothduring production and operation.

The valve bodies according to the invention are produced from thepre-mixed or completely alloyed powder as follows. At first, the greencompact is produced from the powder with the aid of a customary waxserving as lubricant and by applying known compacting pressures toachieve compacts of adequate density. For this purpose the compactingpressure most expediently ranges between 500 and 900 MPa. After pressingthe product is initially dewaxed under a protective hydrogen-nitrogengas blanket at temperatures between 500 and 750° C. and subsequentlysintered in a furnace at a temperature of more than 900° C., preferablymore than 1000° C., and up to 1150° C. Pressures and temperatures inthis case primarily depend on the desired density of the compact andcomposition of the metal powder. Having cooled down the compacts aresubjected to a tempering treatment and further processing steps asrequired.

As mentioned above, valve bodies and valve shafts according to theinvention used for the manufacture of valves for internal combustionengines are produced in separate processing steps and then joined. Saidvalve body is produced by powder metallurgical methods, the shaft end byconventional processes. Body and shaft in this particular configurationcan be joined by friction welding. Following the step of joining thecomponents the valve is subjected to further processing steps.

FIG. 1 depicts a valve body 1 that has been made by powder metallurgicalmethods and is intended for a butt-joining connection with a shaft 2.

EXAMPLE:

For a valve body according to the invention metal powder of thefollowing chemical composition by weight has been used: 0.9% carbon,8.2% molybdenum, 4.8% tungsten, 1.4% vanadium, 0.42% phosphorus, 3.2%chromium and 1.2% sulfur.

Other elements were present in the mixture in the amount of about 1.9%with the remainder being iron.

A sintered compact of sintered molybdenum-phosphorus steel was obtainedthat had a density of 6.9 g/cm³. When subjected to high surface loadsthe compact showed good wear resistance and in the structure variousfinely dispersed carbides in a tempered martensitic matrix with anembedded solid lubricant.

What is claimed is:
 1. A valve body produced by a powder metallurgicalmethod from a metal powder that is at least to some extent, in the formof a completely alloyed PMoFe powder, and having a high resistance totemperatures and wear, comprising the following composition by weight:0.5% to 2.0% C; 5.0% to 16% Mo; 0.2% to 1.0% P; 0.1% to 1.4% Mn; 0% to5% Cr; 0% to 5% S; 0% to 7% W; 0% to 3% V, <2% of other elements withthe remainder being Fe.
 2. The valve body of claim 1, wherein said bodyhas a density of at least 7.5 g/cm³.
 3. The valve body of claim 1,wherein the powder is used in the process in atomized condition.
 4. Thevalve body of claim 1, wherein the following composition by weight isused: 0.5% to 2.0% C; 5.0% to 14% Mo; 0.2% to 1.0% P; 0.1% to 1.2% Mnmax. 0.5% Cr, max. 0.4% S, <2% other elements, remainder Fe.
 5. Thevalve body of claim 1 and having a density of at least 7.7 g/cm³.
 6. Thevalve body of claim 1, wherein said body is compacted by means of theliquid-phase sintering process.
 7. A valve, comprising a valve body ofclaim 1 and a shaft made by conventional methods and connected by a buttjoining process.
 8. The valve according to claim 7, wherein the valvebody and valve shaft are connected by friction welding.